This invention relates to the art of integrated circuits, and more particularly to the art of digital semiconductor memories. Basically, a digital semiconductor memory is a circuit on a semiconductor chip which operates to store digital information. Over the last ten years, the number of bits which can be stored in a single semiconductor memory chip has continuously grown; and present day memory chips store up to 65,536 data bits. Typically, these memory chips are utilized in conjunction with digital logic chips to form a digital computer or special purpose controller.
Now in the prior art, both read-write semiconductor memories and read-only semiconductor memories exist. A read-write memory is one where the stored data bits are not fixed in a predetermined pattern but instead are electrically alterable to any desired pattern. That is, during the memory's operation, any desired data pattern can be written into the memory and read therefrom by the application of appropriate memory control signals.
By comparison, in a read-only memory, the data bits are fixed. That is, the data bits in a read-only memory are written only one time in a predetermined pattern; and thereafter, those data bits can be read from the memory but they cannot be altered by another write operation.
Both the read-write memory and the read-only memory have their own particular advantages and disadvantages. For example, a read-write memory is suitable for use as a "scratch-pad" from which information can be retrieved and sent to an arithmetic unit; and the results can be stored back in the read-write memory. On the other hand, the number of bits that can be stored in a read-write memory is generally less than the number of bits that can be stored in a read-only memory. Further, data in a read-write memory is volatile in that it is destroyed whenever power is inadvertently lost on the chip.
Thus, depending upon the particular application, a digital system will include in its architecture either a read-write memory, or a read-only memory, or both. For example, a digital system might include 64,000 words of read-write memory and 64,000 words of read-only memory.
Consider now a digital system which includes both read-write memory and read-only memory; but the digital system operates such that information from the two memories is never accessed at the same time instant. For example, the read-only memory could contain digital information which is utilized only during a power-on sequence to initialize the rest of the digital system. Then, after this initialization sequence, the read-write memory could be used as a scratch-pad for data which is operated on by an arithmetic unit.
One problem with such a system is that it is undesirably expensive; and another problem is that it is undesirably large in physical size. This is because separate memory chips are provided to perform the read-only and read-write memory functions, even though those two functions are not performed at the same time. Thus to reduce the cost and physical size of the system, it would be highly desirable to provide a single semiconductor memory which in one mode operates as a read-only memory and in another mode operates as a read-write memory.
Accordingly, a primary object of this invention is to provide a semiconductor memory wherein one fixed set of data is stored which can be accessed in a read-only mode, and wherein a variable set of data can also be stored and accessed in read-write mode.